C-Mod Transport Program

1. C-Mod Transport Program PAC 2007 Presented by Martin Greenwald MIT – Plasma Science & Fusion Center 1/24/2007

2. Introduction – Programmatic Focus • Upgrades Offer Opportunities Aligned With Transport Interests • LHCD – efficient, high-power, off-axis current drive • Direct manipulation of magnetic shear • Cryopump  extend range of collisionality • Improvements in profile and fluctuation diagnostics • Transport is a broad topic – where do we focus (in general)? • Where C-Mod has unique capabilities, runs in unique regimes or observes unique or unusual phenomena. • tei<<tE, Ti = Te , n* range, no core particle or momentum sources • Where we can make important comparisons with other devices especially in support of ITER

3. Where Do We Focus In Particular?Transport Themes For 2007 Campaign • Tests of standard models for ion transport: • Effects of magnetic shear (exploitation of LHCD) • Effects of collisionality (exploitation of cryopump) • Investigations into other transport channels including • Momentum – predict rotation with low torque, low r* ? • Electron energy - no ignorable energy channel • Particle and impurities – density profiles with no core source? • H-mode physics • Thresholds - especially role of edge flows, topology • Pedestal structure – width and height, rotation and Ti profiles • Pedestal relaxation – especially small/no ELM regimes • ITB physics • Turbulence suppression mechanism (not ExB) • Control of barrier location and strength Ti Fusion Performance – underlying physics of ITER baseline scenario – underlying physics of advanced scenarios

4. Transport Is An Important Element For Program Thrusts and Many Of Our Topical Groups • Tests of standard models for ion transport • Effects of magnetic shear (edge, baseline & advanced scenarios) • Effects of collisionality (edge) • Investigations into other transport channels including • Momentum (baseline scenarios, MHD and edge) • Electron energy (advanced scenarios) • Particle (baseline & advanced scenarios and edge) • H-modes • Thresholds (baseline scenarios) • Pedestal studies (baseline scenarios) • Pedestal relaxation (baseline scenarios & MHD) • ITBs (advanced scenarios)

5. Comparisons With Theory And Modeling Form A Critical Part Of The Program • Prediction and control are the ultimate goals of transport studies. • Experiments and theory have progressed to the point where meaningful, quantitative tests are being made. • Theory plays critical role in motivation and design of most of our transport experiments - synthetic diagnostic development • Validation of codes is an emerging theme in the transport community • We have close collaborations with theory and modeling groups at MIT and elsewhere – these will continue. • C-Mod contributions for upgraded local Beowulf cluster (will allow nonlinear gk runs locally)

6. Exploitation of LHCD: Steady-state Control Of Magnetic Shear • With Te ~ Ti , g > wExB , ZEFF << Z, R/Ln < R/LT; magnetic shear (Ŝ) is one of the few parameters predicted by drift-wave simulations to determine R/LT. • We can exploit LHCD to allow direct manipulation of shear. • High Priority Experiment: Test ITG models by evaluating change in R/LT and fluctuations as we modify Ŝ (lowish density L-modes with current LH power levels). • Note additional work on effects of magnetic shear in pedestal and edge studies From linear ITG calculations – IFS-PPPL model Kotchenreuther et al, 1995

7. These Experiments Underpin ITB, Hybrid-Mode Research Area • Goal: Profile control via transport control • At weak or reversed shear, instability growth rates are predicted to be much lower (also - suppression of sawteeth with q > 1) • With Ŝ near or below 1 (and weak ExB flow shear!), can we create and maintain ITBs with strong central heating? • Can we produce simultaneous barriers in electron and ion transport channels? • Can we increase core gradients (and bootstrap current)? • Control barrier strength and position? • What are effects of rational q surfaces? • For AT we need to understand the underlying transport physics

8. Plasma Profiles Drift Waves Zonal Flows Collisional Damping Collisionless Damping Impact Of Collisionality On Transport Has Become An Important Issue • Physics Issues – nonlinear regulation of turbulence • ITG - effects through change in electron dynamics • Reduction of ITG instability drive predicted to be more important than zonal flow damping? • TEM – Drives and dissipation? Effects on particle transport and density profile? • Practical Issue: density profile

9. Angioni PRL 2003 C-Mod Test Collisionality Effects on Particle Transport/Density Profile With No Particle Source • ITER Interest – better fusion performance with moderate density peaking • Results from ASDEX, JET at low n* • We’ve begun work on this (in a high-triangularity shape) • High Priority Experiment: Exploit cryopump to broaden experimental base, scan n*, d, q95 (ITPA CDB-9) • Pedestal studies carried out at same time Addition of C-Mod data suggest that neff is appropriate scaling variable rather than n/nG (These are strongly correlated, especially on any given machine) – Good news for ITER

10. C-Mod to match (2007) DIII-D to match (2007) Momentum Transport and Self-Generated Rotation • Rotation is critical for stabilization of turbulence and MHD • But we’re moving toward low torque, low r* regimes (ITER) • What is the origin and scaling of self-generated flows? • What is the role of boundary flows, neutrals? • Is there a steep rotation pedestal? • If so, how is momentum transported in that region? (deGrassie, Rice) Initial C-Mod/DIII-D dimensionless identity experiments have demonstrated match

11. Momentum Transport Research Plans Major diagnostic initiative  HIREX III + NeSox + CXRS • High Priority Expt: Multi-machine studies w/ITPA (in baseline scenarios area) • High Priority Expt: Dimensionless scaling experiments w/DIII-D (in baseline scenarios area) • High Priority Expt: Magnetic braking (in baseline scenarios area) • High Priority Expt: Pedestal rotation profiles and transport + SOL-edge-core coupling • Compare with theory, gk simulations (Ince-Cushman, Rice, Bitter, Hill) New high-resolution x-ray spectrometer (HIREX III) with increased radial coverage, resolution, time response (part of MIT/PPPL collaboration)

12. kq ~ 50 cm-1 kq ~ 30 cm-1 Electron Energy Transport • High Priority Expt: PCI upgrades allow localized measurements of fluctuations at k up to 50 cm-1 • This should be adequate for comparison with coupled ITG-TEM-ETG simulations (Waltz & Candy) • Test in electron dominated plasmas • High Priority Expt: Investigate electron transport with localized transient perturbations from LHH. (Lin)

13. Particle and Impurity Transport • ITER • What will density profiles be? Impurity content? Fueling requirements? • Collisionality dependence (Experiment as noted earlier) ? • Relative importance of TEM and ITG in particle transport? • Gyrokinetic studies with gs2, gyro (TEM identified in ITB) • LHCD should enable extension of Tore Supra experiments with no core source or Ware pinch (Use VL = 0 LHCD experiments) • High Priority Expt on poloidal asymmetries in impurity particle densities • We’re rebuilding laser blow-off equipment to enable routine studies of impurity transport – multiple injections per discharge (experiments in 2008)

14. (LaBombard) H-mode Threshold Studies • Physics still uncertain, significant scatter in empirical databases • Role of edge flows in B drift effect opens new avenue to explore • High Priority Experiment: Low density threshold for L-H transition (in baseline scenarios area) • High Priority Experiment: Study slow evolution with improved diagnostics • Especially Er and Ti evolution • High Priority Experiment: Relationship of pressure gradient, collisionality and SOL flows in L-H transition

15. L-H Threshold: Recent Experiments Found “Two-stage” Transition • Te pedestal begins to develop before L-H transition • Two stages clearly defined in fluctuations • Evolution of Er? (new diagnostics in place) (Hubbard)

16. Pedestal/Edge Barrier Structure • Issue: predicting width: Concentrate on role of magnetic shear and collisionality • Shear-related quantities (q, d) are the only ones that effect pedestal width in C-Mod • High Priority Experiment: Role of magnetic shear via scans of q, d, k • High Priority Experiment:Separate magnetic shear and plasma flow effects in SOL transport via inner gap scan (Hughes)

17. Pedestal: Ion Physics • Spectroscopic diagnostics beginning to bear fruit… • High Priority Experiment: Ion channel: measurements of Ti, V, Er in pedestal • Previously mentioned n* scan (Rowan, McDermott) (Marr)

18. Pedestal Relaxation Mechanisms • ELMy regimes may become more important as we move to lower collisionality with cryopump • ELM research (discussed by others) • Work on QC mode will continue • We’ve observed interaction of QC mode structure and X-point location – needs detailed comparison with code (BOUT) (Lin) PC picks up fluctuations in plasma edge (top and bottom) which are reduced when observation chord is near separatrix

19. (Zhurovich) non-ITB ITB ITB: Barrier Formation And Control In C-Mod Understood As Interplay Of Density And Temperature Gradients • ITBs created with off-axis ICRH • Physics scenario supported by experiments and simulations. • ITBs In C-Mod Not Dominated by ExB Stabilization • Formation driven by reduction in LT • ITB forms when region of ITG stability reaches (future) barrier foot • Feedback via Ln ITG(+), TEM(-)

20. Heat pulse slows as it propagates through the barrier ITB Program • High Priority Expt: Verify model for barrier control via TEM turbulence using upgraded PCI measurements • Barrier foot is a particularly interesting region • High Priority Experiment: Measure fluctuations at foot of barrier • Reflectometer • Correlation length measurements • 140GHz channel = 2.4x1020 • Improved Ti profiles with HIREX III • Background Experiments: Prepare ground for barrier experiments w/ modified shear • (Wukitch)

21. Transport - Highlights • Opportunities to exploit this campaign • Facility upgrades: Cryopump and LHCD will allow important parameter variation (n*, S) • Continuing upgrades in core profile and fluctuation measurements • Enhancements to local computing cluster • Areas of key contribution • Direct manipulation of magnetic shear • Control of collisionality • Self-generated rotation and momentum transport • L/H transition physics: especially flows, B effect • Parameter extension relative to low field tokamak (for example in pedestal or ELM studies) • Reactor relevant ITB regimes